1. Field of the Invention
The present invention is directed to a pulse sequence for a nuclear magnetic resonance tomography apparatus wherein the repetition time of the scans is not constant.
2. Description of the Prior Art
Nuclear magnetic resonance tomography devices are known in the art wherein an examination subject is disposed in a fundamental magnetic field, and is further subjected to a sequence of gradient magnetic fields. A RF coil generates a sequence of RF pulses, which result in nuclear magnetic resonance signals being generated within the examination subject. An RF pick-up system acquires the nuclear magnetic resonance signals, from which an image of the examination subject is constructed. By selectively switching the gradient fields on and off during a scan, an image of a selected slice of the examination subject can be obtained.
As used herein, the term "scan" means the execution of a measuring event for producing an individual Fourier projection. A matrix, from which an image is acquired by two-dimensional or three-dimensional Fourier transformation, is obtained based on the measured values of a number of scans, which are repeated with a repetition time.
A non-constant repetition time in the pulse sequences comprising a scan can occur, for example, if the measurements are triggered dependent on movements of the examination subject. For example, such triggering can ensue on the basis of respiration as described, for example, in European Specification 0 117 725 and in the article "Magnetic Resonance Imaging With a Respiratory Gating," American Roentgen Ray Society, AJR 143, pages 1175-1182, Dec. 1984. Triggering based on the patient's ECG signal is also frequently employed for examinations in the region of the heart. The purpose of triggering the scan dependent upon the motion of the patient is to prevent or reduce image artefacts caused by such motion. This means, however, that the repetition time of the scans is no longer constant, but is instead dependent upon the respective motion, such a respiration or heart beat.
The absence of a constancy of the repetition time results in a deterioration of the quality of the NMR images due to eddy currents, as is described in greater detail below.